Magnetic resonance tomography systems are imaging devices that align nuclear spins of the examination subject with a strong external magnetic field in order to create an image of an examination subject and excite the spins into precession around this alignment using an alternating magnetic field. The precession or return of the spins from this excited state to a state with less energy in turn generates a responsive alternating magnetic field, also known as a magnetic resonance signal, which is received using antennas.
With the aid of magnetic gradient fields, a spatial encoding is imprinted on the signals that subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated and a three-dimensional image of the examination subject is provided. The image generated indicates a spatial density distribution of the spins.
In a constant, homogeneous magnetic field B0, the resonance frequency of the nuclear spins falls within a very narrow band (a few hertz to kilohertz) around what is known as the Larmor frequency. A non-homogeneous B0 field leads to a spatial variation in the resonance frequency, and a global change in the strength of the magnetic field leads to the center being moved. If the B0 field changes without the frequency and spectral distribution of the excitation pulses being adjusted, then the excitation of the nuclear spins via the excitation pulse by the B1 field is not sufficient and leads to a weak signal or has spatial gaps. Unwanted changes in the time-dependent magnetic field components, due for example to eddy currents generated by the gradient coils for spatial encoding likewise lead to such effects. In order to overcome this problem, it is possible, for example, to broaden the bandwidth of the excitation pulses, leading, however, to greater radiation exposure of the patient through absorbed radio waves and restricting the maximum output due to limiting values.
Variations in the magnetic field occur among other things as a result of temperature changes in components that have temperature-dependent magnetic properties, such as, for example, the outer shell of the cryostat or of the gradient coils themselves.
Document DE 10 2012 2017 594 B4 discloses a magnetic resonance tomography system including a frequency control device and a temperature sensor that controls a frequency of the excitation pulse as a function of the temperature signal.
The narrow bandwidth and the temperature dependency lead, however, to temperature changes in the milli-Kelvin range becoming relevant. In order to avoid additional noise components through compensating for this, an averaging of the temperature measurement ensues over fairly long periods, subsequently leading to greater frequency shifts when the measured temperature change is converted after an averaging period and thus to artifacts in the imaging.